I use SpeedFan when benchmarking to make sure my temps don't get too high. if you don't need it for continuous readouts and are just curious what the temp is, head to your BIOS cuz the temp is usually in there.
If you want some thing that is really customisable on temperators sencing, then go the hardcano. The big down fall for this item is that they are overpriced for what they do. RRP about AU$160.
However, if you like set your own alarm temps, (especially if you are cover clocking) then it's worthwhile.
It also monitors your fans speeds RPMs. And you can custom what speeds that you want your fans spinning at.
However if you just want some thing that is easy to install and set up, Silverstone Eudemon SST-FP52 Temperature and Fan Controller is really good. I only paid about AU$45 for my one. It monitors 3 fan and temp. sensors. And you can not adjust your alarm temp as it's hard coded into the unit.
There is not as much customisability on this unit, but some people will not find that as a priority.